The present invention concerns a striker device for printers, comprising a printing support, a character-carrying member having individually movable characters, a print hammer which is actuable in an impact sense and direction to cause a selected character of the character-carrying member to strike against the printing support, and an actuating member movable to actuate the print hammer.
European Patent Application No. 0 206 701 discloses a striker device for typewriters in which the actuating member comprises an armature which is fixed to the hammer and which is actuated electromagnetically. The hammer and the armature are biased into a rest position by a return spring and a counterweight and suitable damping elements limit the oscillations of the hammer about its rest position. The weight of the hammer and the armature is about 5 grams and the energy for the striking operation is produced by imparting a substantial speed to the assembly. That device is reliable and fast but the printing thereof is rather noisy.
The striking noise is closely linked to the energy imparted by the hammer and the duration of the impact and, other conditions being equal, the intensity thereof increases with the increase in deformation of the roller and the desired printing strength. The striking noise is characterized by frequencies which rise in inverse relationship to the duration of the impact, and the noise nuisance also increases with increases in those frequencies. The behaviour of the roller can be compared to that of an elastic-dissipative oscillating system. In particular a hammer-armature group of mass `M` which strikes against the roller induces an oscillation, the frequency of which is approximately: ##EQU1## in which `Kp` and `C` represent coefficients of hardness and damping of the roller. In the systems of known type, a reduction in the amount of noise by means of a reduction in the degree of hardness `Kp` and an increase in the mass `M` encountered a limit in the reduction in printing efficiency and in the increase in bulk of the actuating unit.
U.S. Pat. No. 4,668,112 discloses a striker device for a daisywheel-type typewriter in which the intensity of the striking noise is reduced and the frequency thereof is lowered, by increasing the striking time by means of a substantial increase in the striking mass. For that purpose the hammer is pivoted to the carriage of the machine on which the daisywheel is mounted and receives its motion from a transverse bar of large mass, which is pivoted to the frame structure of the machine parallel to the roller and to which the hammer is connected slidably, transversely with respect to the roller. The bar is in turn actuated by a moving-coil electromagnet controlled by a control circuit for prolonging the striking force. That arrangement is of a weighty structure in order to withstand the forces acting at the transverse bar without deformation and it is of high cost due to the complexity and the output power of the pilot control circuits.
U.S. Pat. No. 4,327,639 discloses a striker device for daisywheel-type printers in which the print hammer and the armature are mounted on a support frame structure which is pivotally mounted about an axis on the carriage which is parallel to the roller. The hammer is connected to the support frame structure by means of a highly flexible double leaf spring which, at the moment of the striking action improves the coupling of the character to the hammer. Then, while the armature continues with its movement, the leaf springs activate a second striking action on the part of the hammer in order to improve transfer of the ink from the typing ribbon to the paper, even in circumstances involving low levels of energization of the hammer. The printers which use that device are however noisy.